This invention relates to an acceleration detector and more particularly to an acceleration detector for detecting knocking in an internal combustion engine.
FIG. 1 illustrates one example of an acceleration detector to which the present invention can be applied. This acceleration detector is attached, when in use, to an internal combustion engine A by a bolt B for detecting vibration or knocking of the engine A. The acceleration detector comprises a housing 1 defining an annular cavity 2 therein and an annular acceleration transducer assembly 3 disposed within the cavity 2. The housing 1 comprises a tubular bushing 4 having a through hole 5 and a flange 6. The housing 1 also comprises a ring-shaped outer case 7 bonded to the flange 6 of the bushing 4 so that the cavity 2 is defined therein. The acceleration detector can be attached to a mounting surface C of the internal combustion engine A by the bolt B extending through the central bore 5 of the bushing 4 and thread-engaged with the engine A.
The outer case 7 also has a connector 8 radially outwardly extending from the outer case 7 so that an output terminal 9 can extend through the connector 8 for taking out an output signal from the acceleration transducer assembly 3 disposed within the cavity 2. The acceleration transducer assembly 3 further includes an annular piezoelectric element 11 placed on the terminal plate 10, a washer-shaped terminal 12 including a lead 12a connected to the output terminal 9, an electrically insulating washer 13 disposed on the washer terminal 12, an annular inertial weight 14 placed on the insulating washer 13 and a threaded ring-shaped stop nut 15 thread-engaged with the thread 4a on the tubular bushing 4. The piezoelectric element 11 includes an output electrode 11a and a reference electrode 11b. The insulating washer 13 may be made of a sheet of polyethylene terephtalate (PET), polyphenylene sulfite (PPS) or the like. An electrically insulating tape or tube 16 is placed on the tubular bushing 4 so that the acceleration transducer assembly 3 is insulated from the bushing 4 even when the washer terminal 12 as well as the piezoelectric element 11 are eccentrically assembled.
In order to resiliently support and protect the acceleration transducer assembly 3 within the cavity 2 from undesirable environmental conditions, the remaining space of the cavity 2 of the housing 1 which is not occupied by the acceleration transducer assembly 3 is substantially filled with a resilient filler material 17 of a thermo-setting resin. The filler material 17 must be sufficiently resilient after it is cured to allow the movement of the inertial weight 14 relative to the housing 1 when an acceleration is applied to the inertial weight 14 so that the piezoelectric element 11 generates a voltage signal proportional to the pressure exterted on it by the relative movement of the inertial weight 14 against the piezoelectric element 11.
When in use, the acceleration detector is securely mounted on the mounting surface C of the internal combustion engine A by the bolt B inserted into the central through hole 5 of the housing 1. The acceleration or the vibration of the internal combustion engine A produces the movement of the inertial weight 14 relative to the housing 1, which causes the piezoelectric element 11 to be stressed by the inertial weight 14, whereby an electrical signal indicative of the movement of the inertial weight 14 relative to the engine is generated from the piezoelectric element 11. The electrical signal is provided from the output electrode 11a through the washer terminal 12, the lead 12a and the output terminal 9 to be analyzed to determine as to whether or not a knocking signal which generates upon knocking of the internal combustion engine is involved. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
Since the detection signal from the piezoelectric element 11 is provided with reference to the potential level of the bushing 4 which is at equal potential to that of the reference electrode 11b of the piezoelectric element 11, it is important to establish a good electrical connection between the bushing 4 and the engine A in order to ensure that the bushing 4 is always kept at a reference potential, establishing a proper electrical connection between the bushing 4 and the engine A as shown in FIG. 2. This electrical connection is established by a direct contact between the bottom surface 6a of the flange 6 of the bushing 4 and the mounting surface C of the engine A as well as through the screw B which contacts the inner and the top surfaces 4b and 4c of the bushing 4 on one hand and the threaded portion of the engine A on the other hand.
However, when the acceleration detector is used with an automotive engine, an electrical connection between the bushing 4 and the engine A can be easily broken by rusts and oil film on the contacting surfaces between the bushing 4 and the engine A generated in a hostile environment including moisture and gas. Therefore, it is very difficult to maintain a good electrical connection between the bushing and the engine for a prolonged time period of from several to 10 years for maintaining the reference electrode 11b at the reference potential. When the electrically conducting state at the interface between the bushing 4 and the engine A is broken and the potential of the bushing 4 is not held at the reference potential of the mounting surface of the engine A, the electric circuit between the piezoelectric element 11 and the control unit becomes as illustrated in FIG. 3, where the reference potential of the piezoelectric element 11 is floating and accurate acceleration detection is impossible.